Electronic display devices may be used to display images from multiple video sources. For example, a computer or other target device may receive signals from multiple video sources and combine the signals to produce an image that is communicated to a display monitor. The combination may involve overlaying images from one video source onto images from another video source in order to produce a “picture-in-picture” effect, background graphics, windows and the like.
This overlay process may be performed on a pixel-by-pixel basis. One way to perform such an overlay is by use of a fast blank signal. A fast blank signal is typically a one-bit signal that may be used to indicate to the display device where video from the second source should be overlaid onto video from the first source. For example, when the second video source reaches a certain x, y position or pixel, it transmits a fast blank signal to the target device. The fast blank signal causes the video from the second source to replace the video from the first source for that pixel. Because the primary (first) source and overlay (second) source typically operate independently (e.g., at different frequencies), the sources must be synchronized prior to the overlay process. This may involve primary source acting as a master and providing horizontal and vertical synchronization signals to the overlay source, which acts as a slave. This synchronization and corresponding overlay process is very complicated and difficult, especially in applications requiring frame rate conversion and scaling to different resolutions.
Therefore, it would be desirable to provide a new and improved system and method for overlaying images from multiple video sources on a display device, which allows video from an overlay source to be combined with video from a primary source without receiving synchronization signals from the primary source.